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How Small is an Atom?  A PENNY contains about 2 x 10Ε22 atoms or 20,000,000,000,000,000,000,000  That’s 20 thousand billion billion atoms- over 3,000,000,000,000 times more atoms than there are people on earth.  An aluminum atom has a diameter of about cm.

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What is the Nucleus Made Of?  Protons: positively charged particles in the nucleus.  The mass of a proton is about 1.7 x 10E -24g.  Because atoms masses are so small scientists made a new unit for them.  Protons = 1amu  Neutrons: particles of the nucleus that have no charge.  Neutrons= 1amu  Atomic mass unit (amu): the SI unit used to express the masses of particles in atoms.

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What’s Outside the Nucleus?  Electrons are found outside the nucleus in the electron clouds.  They are negatively charged.  It takes 1,800 electrons to equal the mass of one proton. They are so small they are usually thought to have a mass of almost zero.

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Ion(s)  The charges of protons and electrons are opposite but equal, so their charges cancel out.  Because an atom has no overall charge, it is neutral.  If the numbers of protons and electrons are not equal, then the atom becomes a charged particle called an ION.  An atom that loses one or more electrons is a positively-charged ion.  An atom that gains one or more electrons becomes a negatively-charged ion.

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How Do Atoms of Different Elements Differ?  There are more than 110 different elements… they are all different in some way.  You can tell the difference between atoms of separate elements by the number of protons in the nucleus of an atom (atomic number).  Atomic number: the number of protons in the nucleus of an atom.

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Isotopes  Isotope: atoms that have the same number of protons but have different numbers of neutrons.  You can have an isotope of the same atom.  Ex. Hydrogen atom vs Hydrogen Isotope  Hydrogen atom= 1proton & 1electron  Hydrogen isotope= 1proton, 1nutron, & 1 electron

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Properties of Isotopes  Each element has a limited number of isotopes that are found in nature.  Some isotopes have special properties because they’re unstable. These are RADIOACTIVE.  Radioactive atoms spontaneously fall apart after a certain amount of time.  As they do, they give off smaller particles, as well as energy.

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Properties of Isotopes cont.  Isotopes of an element share most of the same chemical and physical properties. For example, the most common oxygen isotope has 8 neutrons in the nucleus. Other isotopes of oxygen have 9 or 10 neutrons. All three isotopes are colorless, odorless gasses at room temperature. Each isotope has the chemical property of combining with a substance as it burns. Different isotopes of an element even behave the same in chemical changes in your body.

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Telling Isotopes Apart  You can tell each isotope apart by it’s mass number.  Mass Number: the sum of the protons and neutrons in an atom.  Electrons are not included in an atom’s mass number because their mass is so small.

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Naming Isotopes  You can tell the difference between isotopes by finding how many neutrons they have.  To identify a specific isotope of an element, write the name of the element followed by a hyphen and the mass number of the isotope.  Ex. Hydrogen-1 or carbon-12  You can find the number of neutrons in an element by using the following equation:  Mass number – Atomic number = number of neutrons

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Calculating the mass of an element  Most elements contain a mixture of two or more isotopes.  Ex. All copper is composed of copper-63 atoms and copper-65 atoms.  Atomic mass: the weighted average of the masses of all the naturally occurring isotopes of that element.  To find the atomic mass:  Multiply the mass number of each isotope by it’s percentage abundance in decimal form.  Add these amounts together to find the atomic mass.

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Example of Finding Atomic Mass  Chlorine-35 makes up 76% of all the chlorine in nature, and chlorine-37 makes up the other 24%. What is the atomic mass of chlorine?  Step 1: Multiply the mass number of each isotope by it’s percentage abundance in decimal form.  (35 x 0.76) =  (37 x 0.24) =  Step 2: Add these answers of the amounts together to find the atomic mass. DON’T FORGET TO LABEL YOUR ANSWER IN AMU’s.

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Forces of Atoms  Forces are “pushes” and “pulls.” You have seen the make-up of individual atoms but we have not discussed the forces acting between more than one atom.  There are four basic forces at work everywhere between and around an atom.  Gravitational force  Electromagnetic force  Strong force  Weak force  These forces work together to give atoms their structure an properties.

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Gravitational Force  Gravitational force acts between all objects all the time.  The amount of gravitational force between objects depends on their masses and the distance between them.  Because the masses of particles in atoms are so small, the gravitational force within atoms is very small.

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Electromagnetic Force  Objects that have the same charge repel each other, while objects with opposite charge attract each other.  This is due to electromagnetic force.  Protons and electrons are attracted to each other because they have opposite charges.  The electromagnetic force holds the electrons around the nucleus.

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Strong Force  Protons push away from one another because of the electromagnetic force.  A nucleus containing two or more protons would fly apart if it were not for the STRONG FORCE.  At the close distances between protons and neutrons in the nucleus, the strong force is greater than the electromagnetic force, so the nucleus stays together.

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Weak Force  The WEAK FORCE is an important force in radioactive atoms.  In certain unstable atoms the neutron can change into a proton and an electron.  The weak force plays a key role in this change.